Modulation system for velocity modulation tubes



March 2, 1943. c. v. LlTTON 2,312,919

MODULATION SYSTEM FOR VELOCITY MODULATION TUBES Filed Sept. 19, 1940 2 Sheets-Sheet INV EN TOR. CHARLES V. L/TTO/V ATTORNEY.

March 2,1943. c. v. L lTT ON 2,312,919.

MODULATION SYSTEM FOR VELOCITY MODULATION TUBES Filed Sept. 19, 1940 2 Sheets-Sheet 2 31 I 17 sou/P05 0F 7'0 noouum'a 0mm may VOLTAGE FREQUENCY OSCILLAT/O/YS INVENTOR. CHAFLES'L/ L/TTO/V A TTORNE Y.

atented 2, i943 MODULATION SYSTEM FOR VELOQETY Charles V. Litton, Redwood City, Caliifi, assignor to International Standard Electric Corporation, New York, N. Y, a corporation oi Delaware Application September re, 1940, Serial No. 357,403

Claims. (or. 179-1715) The present invention relates to modulation systems and more particularly to modulation systems for ultra-high frequency circuits employing velocity modulation tubes.

Velocity modulation tubes have heretofore been proposed wherein a stream of electrons is accelerated longitudinally for the purpose of generating or amplifying high frequency oscillations. Velocity modulation tubes are commonly constructed with a cathode, a focussing electrode, accelerating grids, absorbing grids, resonant chambers connected to these grids, and a conductive loop coupled to one of the resonant chambers.

I have found that it is possible to amplitude or frequency modulate the output of a velocity modulation tube by coupling a variable impedance to a resonant chamber of such a tube and varying said impedance at a modulation frequency rate. In the case of amplitude modulation-I couple a variable impedance to the resonant chamber of an amplifier or modulator tube, and in the case of frequency modulation I couple a variable impedance to the resonant chamber of an oscillator tube. In accordance with my invention, therefore, I modulate the output of a velocity modulation tube by coupling a variable impedance which may be capacitive, inductive or resistive to a resonant chamber of the tube.

It is an object of my invention to frequency modulate the output of a velocity modulation tube.

It is a further object of my invention to amplitude modulate the output of a velocity modulation tube.

Other objects of my invention will :be apparent from the particular description made in connection with the accompanying drawings wherein:

Fig. 1 illustrates in section a velocity modulation tube oscillator whose output is frequency modulated in accordance with one embodiment of my invention;

Fig. 2 illustrates a velocity modulation tube amplifier whose output is amplitude modulated in accordance with one embodiment of my in vention; I

Fig. 3 illustrates a portion of a velocity modu-= lation tube and a variable impedance tube coupled to a conductiveloutput loop of the velocity modulation tube.

In Fig. i I have shown in section a known type of velocity modulation tube comprising a h mmmmv i, an accelerating grid and variable condenser.

.circuit assembly 2, an absorbing grid and circuit assembly 3, and a collector electrode 4. The cathode comprises a filament 5 which is preferably non-inductively wound, an indirectly heated cathode 6, and a focussing shield i.- Electrons from the indirectly heated cathode 6 pass through accelerating grids 8 and 9 and drift column ill which bunch the electrons into spaced electron groups. Absorbing grids ii and i2 extract energy from the bunched electrons. and the electrons are finally collected by the collector electrode 4. Chambers l3 and it are resonant chambers preferably tuned to the operating frequency of the tube. The output of the tube is taken from resonant chamber it by means of loop i5 connected to the lead it. Lead I6 is part of a transmission line and is sealed to the tube by means of the seal ii. The .outer conductor of the transmission line may be attached to the tube by the threaded portion is. For oscillator operation feedback is obtained between the resonant chamber Ml and the resonant chamber it by means of the conductor I9.

According to one embodiment of my invention I provide an over-coupled conductive loop 26 sealed to the resonant chamber it by seal 2!, and I vary the impedance of this loop by varying a variable condenser coupled thereto at a modulation frequency rate. A housing composed of two parts 22 and 23, threaded at M, encloses two conductor plates 25 and 25 which form the The plate 25 is attached to the lead 2? which is a part of the loop 29, and the plate 26 is attached to the movable rod 28. Plate 26 is electrically connected to the housing 23 by means of a flexible loop 29.

The movable rod 28 is attached to the coil 3% which is in the field of a magnet 3i, magnet 36 being either a permanent magnet or an electromagnet. Those skilled in the art will readily recognize that the coil 38 and the magnet 3i form a device which is similar to a loudspeaker device. The coil 30 may be connected to a source of modulating voltage (not shown) and as the modulating voltage is applied to the coil, the coil vibrates in the magnetic held. The yibra= tions of the coil 8% are transmitted through rod 28 to the movable plate 35. Movement'ot the plate 28 with respect to the plate 25 varies the capacity between these plates, and hence the lim pedance oi the loop 2b is varied at the modula tion frequency rate.

As I have previously stated, I have found. thm

variations in the efiective impedance or a come velocity modulation tube cause changes in the output frequency of an oscillator tube, or changes loop conductors other than those shown may be employed for modulating the frequency of operation of the tube.

Fig. 2 illustrates an arrangement for amplitude modulation of the output of an amplifier tube. In this figure it may be noted that the arrangement is essentially the same as that shown in Fig. 1 except that a single loop serves to couple an output circuit to the tube and to modulate the output of the tube. The tube is separately fed from an external source by a lead 55 sealed by the seal 56 to the connector 51. In this case the tube acts as an amplifier tube, and a feedback loop as shown at IS in Fig. 1 is unnecessary. The output loop shown in Fig. 1 may also be omitted, and the lead l6 connected to the lead 21 at 32 serves to connect the output circuit to the resonant chamber I4. The coil 30 in this arrangement, as well as the coil 30 shown in Fig. 1, may be resiliently mounted by any well known means. If desired the modulating arrangement shown in this figure may also be used to modulate the output of an oscillator tube and thus produce frequency modulation instead of amplitude modulation. This result may be achieved by merely providing a feedback means to'the input chamber to produce sustained oscillation.

Fig. 3 illustrates a portion of a velocity modulation tube and a variable reactance tube con-.

nected to vary the effective impedance of the loop 20. The variable reactance tube circuit comprises a tube 33 having a shield grid 34, a control grid 35, an injector grid 36, an anode 31, and a cathode 38. Resistors 39 and 40 are biasing resistors for the control grid and the injector grid, respectively, and an operating potential is supplied to screen grid 34 over a resistor 4|. Condensers 42, 43 and 44 are by-pass condensers. Resistor 45 is a common cathode biasing resistor and condenser 46 is a blocking condenser. Resistor 41 and condenser 48 constitute a phase dividing network for properly phasing the potential applied to the control grid 35. D. C. potential from the source 49 is supplied to the anode 31 over the choke 50.

The operation of the variable impedance or reactance tube is well known in the art. Its operation may be briefly described by stating that with the resistor 41 large in comparison with the condenser 48, the'current through the series circuit 41, 45 is substantially in phase with the voltage between the lead .15 and the outer conductor 5|. However, the voltage across the condenser 4| lags the current through the series circuit by substantially 90 and hence the current in the tube plate circuit will lag the current through the condenser 45 by substantially 90. This lagging plate current will act as an inductance across the lead I5 and the conductor 5| and the effective value of the inductance maybe varied by varying the amplification through the tube. In the present case the amplification may be varied fore, by applying a modulating voltage to the grid 36 across the resistor 40, the inductance may be made to vary at a modulating frequency rate.

The outer conductor 5| of the output transmission line is clamped to the threaded portion 18 by a threaded nut 52. Washer 53 insulates the coupling lead 54 from the outer conductor 5|. The variable impedance tube connected between the lead l5 and the outer conductor 5| acts as a variable impedance similar to the variable capacity shown in Figs. 1 and 2 and acts to vary the effective impedance of the conductive loop 20, thereby modulating the frequency of operation of the velocity modulation tube.

The modulating arrangements shown in Figs. 1 and 3 may also be employed with amplifier tubes to produce amplitude modulation of the output energy of the tubes. In this case the tubes may be fed from external sources of high frequency oscillations. The feedback loop may then be eliminated.

It should also be noted that the coupled loop may be considered as 'a variable impedance coupled indirectly to the resonant chamber, and it is further possible to couple a variable impedance directly to said resonant chamber without any intervening coupling means.

While I have described particular embodiments of my invention for purposes of illustration, it will be understood that various modifications and adaptations thereof may be made within the spirit of the invention as set forth in the appended claims.

What is claimed is:

l. A modulation system comprising a velocity modulation tube, a resonant chamber associated with said tube for regulating the frequency of operation of said tube, coupling means coupled to said resonant chamber, and means for altering the velocity modulationinto a different form of modulation by varying the effective impedance of said coupling means comprising a variable condenser coupled to said couplin means, and means for varying the capacity of said condenser at a modulation frequency rate.

2. A modulation system comprising a velocity modulation tube, a resonant chamber associated with said tube for regulating the frequency of operation of said tube, a conductive loop coupled to said resonant chamber, a first condenser plate connected to said loop, a second condenser plate in capacity relation with said first plate, means for producing a magnetic field, a movable coil attached to said second plate and positioned in said field, and means for applying amodulating potential to said coil.

3. A frequency modulation system comprising a velocity modulation tube comprising a source of electrons, means for bunching said electrons into spaced electron. groups, absorbing grids, a resonant chamber coupled to said grids, and means for collecting said electrons, and means for converting the output of said tube into a frequency modulated wave, said converting means comprising a variable impedance coupled to said resonant chamber, and means for varying said impedance at a modulation frequency rate.

4. A frequency modulation system comprising a velocity modulation tube comprising a cathode,

by varying the voltage on the grid 36. There- 7 accelerating grids, a first resonant chamber coupled to said grids, absorbing grids, a second resonant chamber coupled to said absorbing grids, output coupling means coupled to said second resonant chamber, feedback means connected between said first and second chambers,

converting means comprising a conductive loop over-coupled to said second resonant chamber and means for varying the effective impedance of said loop at a modulation frequency rate.

5. A frequency modulation system comprising a velocity modulation tube comprising a cathode,

g'rids, output coupling means coupled to said second resonant chamber, feedback means connected between said first and second chambers, and means for collecting electrons from said cathode, and means for converting the output of said tube into a frequency modulated wave, said converting means comprising a variable impedance coupled to said coupling means, and meansfor varying said impedance at a modulaaccelerating grids, a first resonant chamber 30 $1011 frequency ratecoupled to said grids, absorbing grids, a second resonant chamber coupled to said absorbing CHARLES V. LTTTON. 

